The present invention relates generally to the field of medical imaging and more specifically to the field of cardiac imaging by computed tomography. In particular, the present invention relates to the selection of reconstruction projection data to minimize motion artifacts.
Computed tomography (CT) imaging systems measure the attenuation of X-ray beams passed through a patient from numerous angles. Based upon these measurements, a computer is able to reconstruct images of the portions of a patient's body responsible for the radiation attenuation. As will be appreciated by those skilled in the art, these images are based upon separate examination of a series of angularly displaced projection images. A CT system produces data that represents the line integral of linear attenuation coefficients of the scanned object. This data is then reconstructed to produce an image, which is typically displayed on a cathode ray tube, and may be printed or reproduced on film. A virtual 3-D image may also be produced by a CT examination.
CT scanners operate by projecting fan shaped or cone shaped X-ray beams from an X-ray source that is collimated and passes through the object, such as a patient. The attenuated beams are then detected by a set of detector elements. The detector element produces a signal based on the attenuation of the X-ray beams, and the data are processed to produce signals that represent the line integrals of the attenuation coefficients of the object along the ray paths. These signals are typically called projections. By using reconstruction techniques, such as filtered backprojection, useful images are formulated from the projections. The locations of pathologies may then be identified either automatically, such as by a computer-assisted diagnosis (CAD) algorithm or, more conventionally, by a trained radiologist. CT scanning provides certain advantages over other types of techniques in diagnosing disease particularly because it illustrates the accurate anatomical information about the body. Further, CT scans may help physicians distinguish between types of abnormalities more accurately.
Cardiac imaging, such as for the assessment of coronary artery stenosis, using CT imaging techniques presents certain problems, however, due to the dynamic nature of the heart and the fine structures of the coronary vessels. The volume of the heart changes drastically during systole and during the rapid inflow of blood into the ventricles. High temporal resolution is generally desired to freeze the heart motion, while high spatial resolution is needed to identify the moving coronary vessels and the stenotic lesions.
To avoid the imaging problems associated with these substantial volume changes, it is generally desirable to acquire the projection data for image reconstruction during a prescribed phase of interest, typically the end-diastolic phase of the cardiac cycle, when the heart volume is relatively constant. Unfortunately, the mechanical gantries typically available in CT systems do not rotate fast enough to capture a motion-free volume rendering of the heart at various heart rates. These two constraints, selecting a reconstruction data set with the desired cardiac phase and achieving the desired temporal resolution, may be difficult to satisfy simultaneously.
A conventional reconstruction algorithm compensates for these problems by defining the prescribed phase of interest as a percentage of the cardiac cycle for the whole cardiac volume. The reconstruction algorithm therefore positions reconstruction windows, corresponding to the projection data to be analyzed, at prescribed increments from the measured R-peaks in the cardiac cycle. Axial image slabs are generated using the reconstructed image data such that each slab comprises a set of one or more images generated at the same phase of the same cardiac cycle. The number of the images comprising the set is determined by the heart rate and the associated pre-selected table speed, i.e., the linear displacement of the subject. The resulting image slabs, when associated together in order, comprise the desired cardiac volume rendering.
The reconstruction algorithms do not, however, account for changes in cardiac motion at different heart rates or for cardiac volume changes within the same heart cycle. Instead, the reconstruction window is specified by the algorithm at prescribed increments, without accounting for the subsequent R-peak, the P-wave, or to the QT interval of the patient's heart cycle. As a result, if the patient's heart rate changes or if beat irregularities are present, the reconstruction window may be specified outside of the prescribed phase of interest, such as over a T- or P-wave. When this occurs, the image slabs comprising the image of the cardiac volume may be shifted or offset in the coronal and sagittal views, producing phase misregistration artifacts. The so-called “phase misregistration” artifacts occur when successive reconstructed slabs correspond to cardiac cycles at different heart rates, resulting in one slab that is derived at a different state of the cardiac cycle than its neighbors.
One method of addressing this problem is to allow the operator to manually visualize the reconstruction at different phases and to manually select those reconstructions that result in the lowest amount of artifacts in the reformats of the axial data. This manually generated volume of data is then used to construct the cardiac images used for analysis and diagnosis. The method, however, is operator intensive and subject to subjective determinations. A method of addressing this problem, which is less subjective, and less operator intensive is desirable.